A bar of soap is one of many items that a toilet bowl is not designed to receive. If one does end up clogging your toilet, don't try to just flush it down, or you might succeed only in pushing it just a little farther out of your reach, while it continues to clog the toilet. Instead, remove it.
How It Happens
Because a toilet is normally situated close to a sink or bathtub, and because of how slippery wet soap is, it's easy for a bar of soap to slip out of your hands and into the toilet bowl by accident. It also happens on purpose, when small children drop soap into the toilet. Help avoid both of these by keeping the toilet seat lid closed when you're not using the toilet. This prevents soap and other items such as cell phones from inadvertently dropping into the toilet bowl drain, and it discourages people from viewing the toilet as a waste basket where they can just toss and flush foreign objects that will clog it.
What to Do First
Even if the bar of soap slides out of view down the toilet bowl drain, it's still possible that it hasn't maneuvered through the toilet's curved trapway if it hasn't been flushed. Put on rubber gloves and reach into the bowl drain to grasp and pull the soap out. Or, straighten out a wire coat hanger and carefully poke it into the drain until you can pull the soap out.
Bringing the Soap Up
If the toilet has already been flushed, a plunger might not provide enough suction to dislodge the soap from the trapway, but you can try. Place the plunger's rubber cup totally over the drain and push its shaft down and back up rapidly, about 12 times. Try this, but don't be discouraged if it's not effective. Insert the curved end of a closet auger into the bowl drain, and twist the handle clockwise to send the auger's cable snaking through the toilet's trapway. Keep a bucket nearby to toss the soap into if you hook the soap on the end of the cable and drag it back up out of the drain. Or, toss the retrieved bar of soap into the waste basket. Most likely, the auger will break the soap up into smaller bits that will flush down the pipe and allow you to pull out a portion of the bar of soap. Just remember not to flush the toilet until after you have removed or broken apart the soap, or the bowl might back up and overflow.
Lifting the Toilet
If using a closet auger fails, shut off the water to the toilet by twisting the shut-off valve completely clockwise. Flush the toilet and suck out all the water from both the tank and the bowl using a wet/dry vacuum. Remove the nuts from the bolts in the toilet base with a wrench. Lift the toilet from the floor and place on cardboard or newspaper padding. Shine a flashlight into the toilet drain outlet from underneath it. Also look at the area around the flange on the floor, which the toilet sits on, if the soap managed to make it just past the bowl trapway. Remember to position a new wax ring onto the flange before you place the toilet back on it.